Detection of the Splashback Radius and Halo Assembly bias of Massive Galaxy Clusters

TL;DR

This paper detects the splashback radius in galaxy clusters and provides strong evidence for halo assembly bias, revealing discrepancies with simulation predictions and suggesting complex cluster dynamics.

Contribution

It presents the first significant detection of halo assembly bias using cluster-photometric galaxy cross-correlations and compares observed splashback radii with simulation expectations.

Findings

Detection of the splashback radius in SDSS galaxy clusters.

Evidence for halo assembly bias at 6.6-sigma significance.

Observed splashback radii are smaller than simulation predictions.

Abstract

We show that the projected number density profiles of SDSS photometric galaxies around galaxy clusters displays strong evidence for the splashback radius, a sharp halo edge corresponding to the location of the first orbital apocenter of satellite galaxies after their infall. We split the clusters into two subsamples with different mean projected radial distances of their members, $\langle R_{\rm mem}\rangle$, at fixed richness and redshift, and show that the sample with smaller $\langle R_{\rm mem}\rangle$ has a smaller ratio of the splashback radius to the traditional halo boundary $R_{\rm 200m}$, than the subsample with larger $\langle R_{\rm mem}\rangle$, indicative of different mass accretion rates for the two subsamples. The same cluster samples were recently used by Miyatake et al. to show that their large-scale clustering differs despite their similar weak lensing masses, demonstrating strong evidence for halo assembly bias. We expand on this result by presenting a 6.6-$\sigma$ detection of halo assembly bias using the cluster-photometric galaxy cross-correlations. Our measured splashback radii are smaller, while the strength of the assembly bias signal is stronger, than expectations from N-body simulations based on the $\Lambda$-dominated, cold dark matter structure formation model. Dynamical friction or cluster-finding systematics such as miscentering or projection effects are not likely to be the sole source of these discrepancies.